tcp: annotate data-race around challenge_timestamp

challenge_timestamp can be read an written by concurrent threads.

This was expected, but we need to annotate the race to avoid potential issues.

Following patch moves challenge_timestamp and challenge_count
to per-netns storage to provide better isolation.

Fixes: 354e4aa3 ("tcp: RFC 5961 5.2 Blind Data Injection Attack Mitigation")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>